The present invention relates to a system for controlling a lock-up clutch of a torque converter followed by an automatic transmission.
Hydraulic systems for controlling a lock-up clutch of a torque converter followed by an automatic transmission are well known. A representative one is disclosed in JP 61-96259, for example. This lock-up clutch controlling system comprises a lock-up control valve and a range control valve which are related such that the lock-up clutch is ready for activation during operation with the highest speed ratio of each of two selectable operation ranges. That is, in the case where the fourth speed ratio is an overdrive, a lock-up clutch becomes ready for activation during operation with the fourth speed ratio only when a predetermined operation range where an engine brake is not used is selected. On the contrary, when another operation range is selected by pressing an overdrive inhibitor button where the engine brake is used and the fourth speed ratio is prohibited, the lock-up clutch becomes ready for activation during operation with the third speed ratio only. Thus, the lock-up clutch becomes ready for activation during operation with the highest or third speed ratio only when this operation range where the overdrive is inhibited is selected.
However, this known lock-up clutch controlling system has a problem that the installation space for the range control valve is difficult to find and the range control valve is as expensive as the other component valves. This increases size and manufacturing cost of the whole hydraulic control system employing the lock-up clutch controlling system. Describing further in detail, the range control valve is constructed and arranged such that it permits the passage of a third speed ratio hydraulic pressure in the case where a fourth speed ratio prohibition hydraulic pressure is not available, while it prohibits the passage of the third speed ratio hydraulic fluid pressure in the case where the fourth speed ratio prohibition hydraulic pressure is available. The hydraulic fluid pressure delivered by the range control valve acts on the lock-up control valve in opposed relationship with the third and fourth speed ratio hydraulic pressures, controlling a shift of the lock-up control valve. This results in providing the above-mentioned operation. However, the fact that the range control valve has to be provided separately, as a single additional valve, only for the purpose of selectively allowing supply or discharge of the third speed ratio hydraulic pressure to and from the lock-up control valve causes the above-mentioned problem.
An object of the present invention is to improve a lock-up clutch controlling system such that the above-mentioned function is provided without the range control valve.